The invention relates to a fluid friction clutch which includes a working space and a storage space separated by an intermediate wall in a housing for clutch fluid. A rotating clutch disk is disposed in the working space and clutch fluid is selectively guided from the storage space into the working space to drivingly connect the clutch disk and housing. The intermediate wall is provided with clutch fluid return bores in the region adjacent the outer circumference of the clutch disk. Damming bodies are provided in the rotational direction behind these bores for return guidance of the clutch fluid into the storage space.
Fluid friction clutches of this general type are known (DE-OS No. 27 50 520). The damming bodies are there formed as flaps which are movable in the rotational direction of the clutch disc by means of the pressure of the fluid behind the return bores. It is a disadvantage with this arrangement that such fluid friction clutches are very difficult to influence in their control characteristics. Thus, when this type of clutch is arranged for the driving of an axial fan which is arranged for the cooler or radiator of a motor vehicle, the fan is accelerated very rapidly to the full driving rotational speed when the cooling air or cooling water temperature achieves a certain value, and this fan speed is maintained for a certain time, also when the temperature of the cooling medium in the meantime has returned to a lower level. This is true also for other known constructions (Visco-Kupplung SAAB SCANIA, T 20 MUL/EDUL), wherein a so-called "swimming" damming body is provided in a ring groove at the circumference of the driven clutch disk, which body is slidable in the circumferential direction between stop pins or protrusions. The unnecessary continued running of the fan with lowered temperatures leads to an undesired reduction in the overall efficiency level.
The invention is based upon the problem of providing a fluid friction clutch of the above-mentioned kind, but with control characteristics whereby the fan rotational speed increases as constantly as possible with increasing temperatures of the cooling medium and with decreasing temperatures of the cooling medium decreases along a similar characteristic velocity curve.
The solution of this problem according to the invention includes the provision of at least two return bores displaced from one another in the circumferential direction of the intermediate wall, and the provision of damming bodies behind the return bores in the rotational direction of the clutch disk, which damming bodies are provided with a guiding surface for the clutch fluid, wherein said guiding surface extends diagonally toward the inside in the clutch disk rotational direction radially inside of the return bores and extends oppositely to the rotational direction of the clutch disk in the region radially outside of the return bores with an outwardly extending approximately tangentially disposed protrusion. Through the arrangement of the two return bores with the associated damming bodies, the return flow of the clutch fluid in comparison to the through flow is brought into such a circulation that with increasing temperatures of the cooling medium an approximately constant increase in the rotational speed of the fan is achieved. Further, with the corresponding increased sensitivity because of the additional return bore and the damming body configuration, the fan speed is reduced to the empty clutch rotational speed with a very flat hysteresis characteristic during conditions of decreasing temperature of the cooling medium. The new configuration of the damming bodies according to the invention assures that the fan only then runs with the driving disk when a cooling load is present. This results in an improved efficiency level and also to a certain corresponding reduction in the fuel usage for the combustion motor/cooler combination.
It is advantageous according to especially preferred embodiments if two diametrically oppositely disposed return bores are provided in the intermediate wall and the guide surfaces at the damming bodies are respectively configured as flat surfaces which extend along lines connecting two points at different radii, wherein the point at the smaller radius is displaced in the rotational direction of the clutch disk with respect to the other point. It is also advantageous if the line connecting these points is inclined to a tangent extending adjacent the return bore at an angle of at least 35.degree. and at most 55.degree., advantageously approximately 40.degree.. It has been shown that with such an inclination exceptional results are achieved.
It is further advantageous according to preferred embodiments if the guiding surface extends behind the return bore in the rotational direction of the clutch disk and includes a rounded opening in the inner surface of the protrusion, which opening is provided with a half circular shaped contour extending somewhat concentric to the return bore.
Further objects, features, and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a single embodiment in accordance with the present invention.